Buying it is easier, but is a little more expensive. I would say a good start would be 20-40 gallons. You don't want to go too small. But the size is really up to you. The bigger the better, more water volume= more room for those little errors that occur. Here's a link to a videohttp://youtu.be/aaDDKNA4cn0

Just realized that wasn't the right video, lol. But if you just cruise YouTube and Google and search how to set up saltwater sumps you will learn a whole lot more. I'm a more visual learner myself and have utilized a lot from YouTube vids lol.

I would not use a HOB overflow. Just drill the tank. It's super easy. Like said on the previous page, buy a glass holes overflow kit. Overflow Box 1500 GPH
They come wit the bit and everything you need.

I'll second Mr. X and also add that it's easier to get a quiet overflow with a drilled tank than a HOB. I also have a 75, and did not drill it. If I ever have to break the tank down to move it, it WILL be drilled.

Here's a video I made of my sump and an explanation of how mine works for your perusal.

Thanks for all the help much appreciated. And I want to drill it Bc it would be so much better in the long run. It says on the bottom of the tank I have "tempered glass" is that just for the bottom piece or all piece of glass ?

Okk. I may drill it I don't know yet. This is a long work in progress I'm gunna take my time and do it right. What type of lighting do people recommend? Either I'm gunna go all LEDs or fluorescents with the lunar LEDs Bc I loveeee the look of them tanks with the blue lights and everything glowing. But I wanna do corals so I wanna have a light good enough. And the tanks 48" do I have to get a 48" light?

if I'm not mistaken, those proflex sumps have no room for a refugium AND a skimmer....it's either one or the other. The benefit of a good skimmer far outweighs a 10" x 14" refugium.
You are better off building your own out of a standard aquarium.